1. Field of the Invention
The present invention relates to a semiconductor ring laser gyro incorporating a semiconductor as a light source, and particularly to a semiconductor ring laser gyro including a reflection prism for an optical circuit.
2. Description of the Related Arts
A gyroscope has been conventionally known as a means of measuring the angular velocity of an object. Among others, a ring laser gyro, which utilizes the Sagnac effect to precisely measure the angular velocity, is widely used, particularly in the aircraft and rocket industries. While an He—Ne gas laser is primarily used as s laser light source for the ring laser gyro described above, a semiconductor laser, which is advantageous in reduction of device size and power consumption, is recently used increasingly (refer to, for example, Japanese Patent Applications Laid Open Nos. 2001-50753, 2003-139539 and 2006-319104).
FIG. 8 shows a conventional semiconductor ring laser gyro, in which a semiconductor laser 30, four mirrors 31 to 34, and two interference light (beat light) pickup mirrors 35 and 36 are mounted on a silicon substrate. The semiconductor laser 30 has its both ends provided with an antireflection coating and emits two lights respectively from the both ends (refer to Japanese Patent Application Laid-Open No. 2006-319104). One light emitted from one end of the semiconductor laser 30 takes a path such that the light impinges on the mirrors 31, 34, 33 and 32 in this order and falls incident on the other end of the semiconductor laser 30 while the other light emitted from the other end surface of the semiconductor laser 30 takes another path such that the light impinges on the mirrors 32, 33, 34 and 31 in this order and falls incident on the one end of the semiconductor laser 30. This optical circuit functions as a ring resonator, and a laser oscillation occurs at the both ends of the semiconductor laser 30. The four mirrors 31 to 34 are fabricated by anisotropic etching of a silicon substrate (silicon micromachining technique), and a metal coating or a dielectric multilayer coating is provided (refer to Japanese Patent Application Laid-Open No. 2003-139539, Paragraph 0037). At least one of the four mirrors 31 to 34 is a transmissive mirror adapted to introduce part of the light to the beat light pickup mirrors 35 and 36.
In the semiconductor ring laser gyro described above, when an object rotates about a rotation axis (sensitivity axis) defined by the normal line of the silicon substrate, an optical path difference is generated due to the Sagnac effect between the two paths of the lights traveling in the respective directions opposite to each other, and a beat signal based on an oscillation frequency difference is detected. An angular velocity Ω is calculated by a frequency Δf of the beat signal (refer to Japanese Patent Application Laid-Open No. 2006-319104, Paragraph 0015) according to formula 1 shown below:
                              Δ          ⁢                                          ⁢          f                =                                            4              ⁢              A                                      λ              ⁢                                                          ⁢              L                                ⁢          Ω                                    formula        ⁢                                  ⁢        1            where A is an area enclosed by the ring optical path, λ is an oscillation wavelength of the ring laser, and L is a length of the ring optical path.
The semiconductor ring laser gyro using application of the silicon micromachining technique is excellent in that a plurality of mirrors are fabricated to be positioned with a high precision, but an expensive equipment is required for anodic bonding a silicon substrate and a glass substrate. Also, since the reflectance of silicon has a large wavelength dependence, the wavelength of a semiconductor laser is restricted. Further, a mirror may be provided with a reflection coating for the purpose of increasing a reflectance with respect to a certain wavelength, but an advanced coating technique is required for uniformly applying a metal coating or a dielectric multilayer coating to an etching surface rising vertically on the silicon substrate.
It possibly happens depending on use environment that the semiconductor ring laser gyro receives an impact with a large acceleration, and if the optical axis is shifted significantly by the impact, the ring oscillation is stopped. Also, the ring laser gyro usually prevents the lock-in phenomenon by means of dithering the ring laser gyro relative to the sensitivity axis at a frequency higher than the beat frequency, and the optical axis can possibly be shifted also by the dithering.